1. Field of the Invention
The present invention relates to a bracket angle adjustment mechanism.
2. Description of the Related Art
Heretofore, there has been known one type of bracket angle adjustment mechanism. For example, in a vehicle seat assembly 1 as shown in FIG. 18, the bracket angle adjustment mechanism comprises a first bracket 4 fixed to a seat cushion 2, and a second bracket 5 fixed to a seat back 3, wherein a handle 7 fixed to a control shaft 6 is manually rotated clockwise or counterclockwise to adjust an angle of the second bracket 5 relative to the first bracket 4, i.e. a frontward/rearward reclining angle of the seat back 3 (see, for example, Japanese Patent Publication No. 63-47443: Patent Publication 1).
More specifically, the first bracket 4 includes an external-tooth gear 4a, and the second bracket 5 includes an internal-tooth gear 5a which has a larger number of teeth than that of the external-tooth gear 4a. The control shaft 6 has an inward end supported by a central hole 5b of the second bracket 5.
The bracket angle adjustment mechanism further includes a pair of wedge members 11A, 11B and a spring member 12. The pair of wedge members 11A, 11B are fitted in an eccentric space 10 which is defined between an inner peripheral surface of a large-diameter hole 4a formed in a central region of the external-tooth gear 4a and an outer peripheral surface of a small-diameter shank 9a integral with or constituting a central portion of the internal-tooth gear 5a (in this conventional example, the small-diameter shank 9a corresponds to a given circumferential portion 9a of a follower disk 9 fixed to the control shaft 6), when respective portions of the external-tooth gear 4a and the internal-tooth gear 5a are engaged with one another. The spring member 12 is interposed between the pair of wedge members 11A, 11B in the eccentric space to apply a biasing force to each of the wedge members 11A, 11B in a wedging direction, i.e. in a direction allowing each of the wedge members 11A, 11B to be wedged between the inner peripheral surface of the large-diameter central hole 4a and the outer peripheral surface of the small-diameter central shank 9a. The control shaft 6 is adapted to move a wedged-state releasing portion (which corresponds to a follower protrusion 9b formed in the follower disk 9) located between respective wedging ends of the wedge members 11A, 11B. In FIG. 18C, the reference numeral 13 indicates a cover plate fixed to the second bracket 5. The cover plate 13 extends to cover the external-tooth gear 4a of the first bracket 4, and has a bearing portion 13a supporting an outward portion of the control shaft 13a. 
In an operation for adjusting an angle of the second bracket 5 relative to the first bracket 4, i.e. a frontward/rearward reclining angle of the seat back 3, the handle 7 is manually rotated to rotate the control shaft 6. In conjunction with the rotation of the control shaft 6, the wedged-state releasing portion 9b is rotated to move the pair of wedge members 11A, 11B together with the spring member 12 circularly in the eccentric space 10, so that the small-diameter shank 9a is eccentrically moved relative to the large-diameter hole 4b to allow an engagement position of the internal-tooth gear 5a relative to the external-tooth gear 4a to be changed.
In connection with this type of bracket angle adjustment mechanism, there has also been known a technique intended to provide enhanced stability and operability under load by determining respective diameters of a reference circle and an addendum or tip circle of the external-tooth gear 4a or the internal-tooth gear 4b having an involute tooth profile, or by forming a an arc-shaped portion in a tooth tip thereof (see Japanese Patent Publication No. 07-79740: Patent Publication 2).
In the bracket angle adjustment mechanism as disclosed in the Patent Publication 1, there remain much needs to be improved. As one of the problems, when the external teeth of the external-tooth gear 4a with an involute tooth profile are rotated relative to the internal teeth 5a of the internal-tooth gear with an involute tooth profile, a component force is generated in a direction causing release of the engagement therebetween, and an operating force necessary for rotating the control shaft 6 is increased due to the component force.